Here are my thoughts:
Plug in the two first interval points (100 and 500) to your equation in F and get the two answers. Then, subtract the answer you got from 500 and 100 to find the difference, or C(500) - C(100). After you have that, you know that both intervals changed by 400, so your average rate of change should be [(C(500) - C(100)) / 400]. If you plug in those values, you'll get your answer. :)
As far as I goes, it seems like they're looking for the cruising speed as it relates to the length of a ship. It gives you the fact that the longest ship ever built was around 1500 feet, so that's going to be your max in your domain. Given that the first number in the interval of the first problem is 100, I would go with that for the minimum value for your domain. I have a feeling you can figure out what to do with those now. Just plug the minimum value and the maximum value in both equations and find what the min and max value for the range is.
I really hope this helped. It took while to write. haha. Click the thumbs up for me if it helped! Thanks!